The development of expression systems for production of recombinant proteins is important for developing a source of a given protein for research or therapeutic use. Expression systems have been developed for both prokaryotic cells, such as E. coli, and for eukaryotic cells, which includes both yeast (i.e., Saccharomyces, Pichia and Kluyveromyces spp) and mammalian cells. Expression in mammalian cells is often preferred for manufacturing of therapeutic proteins, since post-translational modifications in such expression systems are more likely to resemble those found in a mammal than the type of post-translational modifications that occur in microbial (prokaryotic) expression systems.
Transcription of eukaryotic genes is regulated by a variety of cis- and trans-acting regulatory elements (reviewed by Dillon and Grosveld, Trends Genet. 9:134; 1993). Two of the best characterized cis elements are promoters and enhancers. Promoters are DNA sequences immediately 5' to the coding sequence of the gene and encompass multiple binding sites for trans-acting transcription factors, forming the basal transcription apparatus. Enhancers are also composed of multiple binding sites for trans-acting transcription factors but can be found far up stream or down stream of coding sequences or even within introns. These elements can also act in an orientation independent manner. The activities of promoters and enhancers can be detected in transient expression systems and contain elements which may or may not be tissue specific; they are vulnerable to position effects when studied in stable cell lines or transgenic animals.
Another category of cis- regulatory elements are ones which are believed to regulate the chromatin structure including, locus control regions (LCR) (Grosveld F., et al., Cell 51:975, 1987), matrix attachment regions (MAR; Phi-Van et al., Mol Cell Biol 10:2302; 1980), scaffold attachment regions (SAR; Gasser and Laemmli, Trends Genet 3:16, 1987), and insulator elements (Kellum and Schedl, Cell 64:941, 1991). These elements are similar to enhancers in that they are able to act over long distances, but are unique in that their effects are only detectable in stably transformed cell lines or transgenic animals. LCRs are also dissimilar to enhancers in that they are position and orientation dependent, and are active in a tissue specific manner. In addition, LCR and SAR sequences arc characterized by A boxes, T boxes and topoisomerase II sites, which are not typically found in enhancer or promoter sequences. (Gasser and Laemmli, supra; Klehr D., et al., Biochemistry 30:1264, 1991).
Internal ribosome entry sites (IRES) are another type of regulatory element that can be found in several viruses and cellular RNAs (reviewed in McBratney et. al. Current Opinion in Cell Biology 5:961, 1993). IRES arc useful in enhancing translation of a second gene product in a bicistronic eukaryotic expression cassette (Kaufman R. J., et al., Nucleic Acids Res 19:4485, 1991).
Several vectors are available for expression in mammalian hosts, each containing various combinations of cis- and in some cases trans- regulatory elements to achieve high levels of recombinant protein in a minimal time frame. However, despite the availability of numerous such vectors, the level of expression of a recombinant protein achieved in mammalian systems is often lower than that obtained with a microbial expression system. Moreover, developing a transformed cell line that expresses high levels of a desired protein often requires time consuming cloning and amplification. Accordingly, there is a need in the art to refine and improve expression in mammalian cells, and to identify elements that can augment expression of recombinant proteins and facilitate the use of mammalian cells in recombinant protein production.